Ok so there's more than 8 items but thats because it shows what you have first.

Get the Gloves first and possibly 1 or 2 health pots.

once you have enough money for the blade and one more glove port back.

finish the second blade and get another glove and boots.

once the money starts to role in get a zeal and boots of swiftness or what you want then save up for a BF.

once you have a BF work on infinity and once thats finished get the Phantom dancer.

now start working on the black cleaver. by then the game shall be over but if its extra long sell one blade and work on bloodthirster.

Now the guide.

Pre game:
in my opinion tele and exaust work best with this build but its your call.

if your premade its always nice for a tank to lane with GP because it gives him more time to harrass with his Q.
Laning against a tank is the worst, try to lane against squishies like tristana or teemo.

mastery wise master Q then w, once w is level 3-4 start with E, of course lv up R when its available.

early game:
don't last hit minions with ur q becuase early on you'll have mana problems so always aim for the squishy. Now this takes time to master but don't spamm q when you have it, my rule of thumb is always have enough mana for his W.

ok so by now being lv 5 you should of ither got ur blades and started to harrass. Now save the mana for ur ult and once lv 6, use it on a weak character.
though his ult never is 100% accurate assume you'll only get 2 hits max from his cannons about 200dmg, don't push it or you'll be very disapointed with the results his ult takes 1.5 second to start so think ahead.

midgame. By now you can do fairly good dmg against those baddies aprox 400-600 depending on where you are. if your well fed then don't be afraid to engage lets say a teemo 1v1 you'll mostlikly out hit him. Keep using ur ult for stealing those kills but only if ur team cannot catch up 2 them in time.

late game.
ok so here is where your number 1. you can take on a 2v1 fight by yourself basically though use your best judgement. i generally start with q then w. then start hitting and repeat, since those baddies think your weak they'll try to fight you 2v1 without running, its there mistake. Infact starting with ur ult Q,w then exauhst is insta death.

Of course in early and mid game you use hit an run tactics with your Q unless those baddies are stupid.

try this build before you rate, you won't be disapointed.

Tips:
use W when morgana or ryze traps u etc or NUNU
E on a minion during a team fight
R once the team fight stars= moral increase more likely to not run from a Teamfight
use ur exhaust during a 1v1 at 50% health etc *they think they killed u but nope*
Tele is for pushing *BDING* or BPing for those blades early on
Quicker you get those blades the more income it generates over time
I tried 3 blades but it isn't as effective

BTW: i never lost to another GP with this build ever, infact most GP's get meki pendant first so they can last hit, once they get this. Rest assured unless they out rune you. be prepared to facepalm in laughter.

PPs: i don't use any crit runes and i've infact out dmg GP's who did. So this build is very useful without runes and hella useful with them.